The proposal to use an externally applied magnetic field in the EDM process dates back to earlier stages in the EDM history. When a magnetic field is applied externally to the region of an EDM gap, the ensuing magnetic flux appears to interact with the high-amperage electrical discharge current flow and cause on media generated in the gap certain dynamic actions which have not yet been fully explained. It has been recognized that these interactions facilitate the production of electrical discharges and allow the effective machining gap spacing to be advantageously enlarged so that an increase in the discharge repetition rate and an enhancement in the removal of machining chips and other products from the gap region may result to promote the stabilized EDM actions and thus to generally improve the EDM process. For the prior art in these particular techniques in which the magnetic field is externally applied in the EDM gap, reference is made to my earlier contributions disclosed in Japanese Patents published under publication No. 29-6942 on Oct. 25, 1954, No. 30-833 on Feb. 11, 1955, No. 30-2943 on Apr. 28, 1955, No. 39-13297 on July 11, 1964, No. 46-11400 on Mar. 23, 1971, No. 46-12520 on Mar. 31, 1971 and No. 54-9759 on Apr. 26, 1979 as well as Japanese Utility Model Registration published under publication No. 31-5790 on Apr. 19, 1956.
In the prior-art proposals, magnetic-field generating means is provided, e.g. one or more coils energized by an external source of direct-current or alternating-current nature and is positioned so as to develop a magnetic flux which is constantly fixed to the electrode system with the resulting magnetic lines of force fixedly traversing the confronting surfaces of tool and workpiece electrodes across which machining actions are in progress. The coil may be wound on the tool electrode or workpiece or both, or alternatively an iron core on which an energizable coil is wound may be located in contact with or in the proximity of the tool or workpiece electrode or both to establish the desired stationary magnetic field.
It has now been found that if it is attempted to gain a uniformity of effects achievable by the exertion of a magnetic field over the entire machining zone or a portion thereof of interest, the use of a fixed magnetic flux or its use alone in a manner according to the prior teaching hardly yields the desired results. Thus, the application of a magnetic flux as positionally fixed often causes an undesirable uncontrolled localization or concentration of electrical discharges, a phenomenon especially noticeable when the workpiece is composed of a ferrous or any other ferromagnetic material. This causes excessive and irregular wear of the total electrode as well as impairment of machining stability and hence a reduction in removal rate.